The present invention relates to a piston having at least two piston ring grooves for a piston, an upper groove angled slightly upwardly when viewed from an inner periphery of the ring toward the outer periphery and a lower groove angled slightly downwardly.
Piston rings are usually received within an annular groove disposed about an outer periphery of a piston used within an internal combustion engine. In turn, the piston is reciprocated within a cylinder of an internal combustion engine. Typically, the piston ring is discontinuous, having two end portions. The end portions are separated from one another to expand the piston ring for insertion into a corresponding groove of the piston. The piston ring is then compressed, bringing the end portions closer together, to install the piston within the cylinder.
A piston compresses fluids such as gases within the cylinder. In an internal combustion engine these fluids are ignited and expand, forcing the piston away from the point of ignition. The outer surface or bearing face of a piston ring in an internal combustion engine is subjected to high temperatures, corrosion, and frictional interaction with the walls of the cylinder.
Typically, the uppermost piston ring acts as a combustion seal, keeping the gases of combustion from escaping away from the combustion chamber. In contrast, the lowermost piston ring acts as an oil control ring, preventing oil from escaping into the combustion chamber.
Piston rings acting as combustion seals typically are formed from a single strip of wire formed into a circle with two opposing ends. They are then given a protective wear resistant coating. Finally, the rings are subjected to cutoff and lapping operations to give final surface finish and geometry of form. It is difficult to ensure that a ring contacts all the way around the mating surface of the cylinder wall, let alone further modifying the ring to provide enhanced operational properties. For example, rings arc known having directional characteristics wherein combustion gases are supposedly subject to improved retention within the combustion chamber as the piston and its retained combustion seal move upwardly during the combustion stroke. However, the alleged advantage of such directional characteristics within the combustion seal is compromised when the piston ring is installed upside down.
Piston ring grooves adapted to receive piston rings acting as combustion seals are known that have an upward tilt as viewed from the inner periphery of the piston toward the outer periphery. However, the art teaches that the tilt must be substantial, on the order of tens of degrees. Moreover, the rings themselves must have a corresponding tilt. For example, in U.S. Pat. No. 2,522,764, the upper and lower walls of the groove and the corresponding surfaces of the ring have an angle of approximately sixty-five degrees (65xc2x0) with the direction of the axis of the piston and, likewise, with the inner face of the cylinder wall. In U.S. Pat. No. 2,292,042, a similar inclination is illustrated.
The rationale for having such a substantial upward tilt in the prior art relates to the belief that if a piston ring is substantially inclined upwardly toward the axis of the piston, more force will be required to force the ring to be expanded against the cylinder wall than if the piston ring were supported horizontally in the piston groove. Thus, wear on the cylinder wall is reduced, and particularly wear resulting from gas pressure fluctuation resulting from the combustion process. The disadvantage of such an approach, however, is that while wear is decreased, the passage of unwanted combustion gases past the combustion seal is increased.
Piston ring grooves adapted to receive piston rings acting as combustion seals are also known that have a downward tilt as viewed from the inner periphery of the piston toward the outer periphery. However, the art teaches that the tilt must once again be substantial, on the order of tens of degrees. Moreover, the rings themselves must have a corresponding tilt. For example, in U.S. Pat. No. 3,237,953, the upper and lower walls of the groove and the corresponding surfaces of the ring have an angle from forty-five to eighty-eight degrees (44 to 88xc2x0) with the vertical face of the cylinder in which the ring and piston reciprocate. In U.S. Pat. No. 1,263,850, a similar inclination is illustrated.
The rationale for having such a substantial downward tilt in the prior art relates to the belief that the inclination tends to cause the ring to snugly press against the cylinder wall on an upward combustion stroke.
Piston rings acting as oil control seals are typically formed from a combination of elements, namely upper and lower rails separated by an expander. In the prior art, they are disposed in the lowermost groove of the piston, the groove having the same angular nature as the combustion seal grooves that are disposed above it.
Therefore, there is no consideration of the disadvantage of having grooves with differing angular tilt or inclination depending on the desired function of the piston ring associated with that groove. Instead, a single gang tool holder and tool path is used to reduce cost and simplify the groove making operation.
An inventive piston assembly includes a piston having a plurality of annular grooves extending inwardly from an outer periphery. The grooves are adapted to receive mating piston rings. The uppermost groove associated with a piston ring acting as a combustion seal has an angled tilt or inclination in a first direction while the lowermost groove associated with a piston ring acting as an oil control seal has an angled tilt or inclination in the opposite direction.
In a preferred embodiment, the angle of inclination for the uppermost groove is upward as viewed from the inner periphery of the piston toward the outer periphery and adjusts for thermal droop of the piston ring received within the groove. The angle of inclination is less than approximately ten degrees (10xc2x0), more preferably less than approximately three degrees (3xc2x0), and most preferably less than approximately a degree (1xc2x0). It may be as little as one-tenth of a degree. The piston ring is preferably non-directional in its bias so that it may be installed without concern that the ring may be upside down. Moreover, an intermediate combustion seal comprising a groove and a piston ring may be disposed between the uppermost groove and the lowermost groove. The intermediate groove has the same angle of inclination as the uppermost groove and is formed using the same forming operation as for the uppermost groove.
The angle of inclination of the groove receiving a piston ring, and collectively acting as an oil control seal, is downward as viewed from the inner diameter of the piston toward the outer diameter. As with the uppermost groove, the angle of inclination of the lowermost groove is in the same preferred ranges. However, in practice, the angle of inclination of the lowermost groove will generally be greater than that of the uppermost groove. The piston ring received in the lowermost groove is typically a three-piece ring comprising an expander with upper and lower rails. The oil control seal is designed to scrape oil from the cylinder wall on the downward stroke of the piston and to hydroplane over the oil on the upward stroke of the piston so as to avoid oil movement toward the combustion chamber.